[mythtv-users] Strange resolution problem

[Stephen Worthington]

On Sat, 4 Jul 2015 06:25:11 -0500, you wrote:

>On Sat, 04 Jul 2015 11:31:00 +0100
>Damian <myth at surr.co.uk> wrote:
>
>> On 04/07/2015 08:29, Alec Leamas wrote:
>> > On 04/07/15 08:57, Andréas Kühne wrote:
>> >> What I would do is check the EDID properties of the projector, also
>> >> perhaps change the settings in mythbuntu to not care about the EDID
>> >> values. I don't know how to do that however.
>> >
>> > I did this long time ago for a monitor with a long cable, long
>> > enough to make EDID fail. The solution was to add a modeline to
>> > xorg.conf, in my case it looks like this (mind the email
>> > linebreaks). It still works.
>> >
>> > Section "Monitor"
>> >     Identifier     "Monitor0"
>> >     VendorName     "IBM"
>> >     ModelName      "ThinkVision L190"
>> >     HorizSync       15.0 - 70.0
>> >     VertRefresh     58.0 - 64.0
>> >     ModeLine     "1280x1024_60.00" 108.9 1280 1360 1496 1712 1024
>> > 1025 1028 1060 -hsync +vsync
>> >     Option         "DPMS"
>> > EndSection
>> 
>> Thanks Alec and Andréas. That certainly seems worth a try!
>> 
>> I don't seem to have a xorg.conf file, but I assume the OS will use
>> one if it finds one.
>> 
>> So, now for the million dollar question ...
>> 
>> How do I know the right settings for a xorg.conf entry for my BenQ
>> W1300 projector at 1920x1080? Those HorizSync, VertRefresh, ModeLine
>> and Option settings look like things that you don't want to get
>> wrong!!
>
>There are modeline calculators, online and probably in Ubuntu Software
>Center.

When the projector is in the right mode, try running

  xrandr -q --verbose

from a terminal window.  That gives you a lot (all?) of the data you
need for a modeline.  The output on my system includes a dump of my
TV's EDID data, which is just effectively a set of modelines to use
with the TV.  It is possible with nvidia-settings to dump the EDID
data to a file, then change xorg.conf so that it uses that file
instead of the data over the HDMI or DVI cable (which my possibly be
being read unreliably).  Then set up xorg.conf to lock the display
mode to the modeline (or modelines) in the EDID data that you want the
projector to use.

It is likely that you actually want to be able to use multiple
different modelines depending on what video you are projecting (50 Hz
for PAL, 60 Hz for NTSC and so on) if your projector can handle the
modes.  Without that, you will get judder when the frame rate of the
projector does not match the frame rate of the video.

The xrandr command can be used to change the current mode (ie select a
different modeline) to test what works and what does not.

EDID data is strange - devices quite often seem to have bad EDID data
that actually does not quite work with them, but X knows about a lot
of the problems and fixes quite a few of them.  In my case, I get
this:

[544296.385] (WW) NVIDIA(GPU-0): The EDID for SONY TV (DFP-0)
contradicts itself: mode
[544296.385] (WW) NVIDIA(GPU-0):     "1920x1080" is specified in the
EDID; however, the EDID's
[544296.385] (WW) NVIDIA(GPU-0):     valid VertRefresh range
(48.000-62.000 Hz) would exclude
[544296.385] (WW) NVIDIA(GPU-0):     this mode's VertRefresh (24.0
Hz); ignoring VertRefresh
[544296.385] (WW) NVIDIA(GPU-0):     check for mode "1920x1080".

in my Xorg.0.log file.  What that means is that the 1920x1080 modeline
data is OK (it does work), but elsewhere in the EDID data the limits
for the vertical refresh settings are invalid and X had to ignore them
to use that modeline.

Playing around with modelines is not entirely safe.  With most modern
video devices, sending an invalid signal to it will just make the
screen go blank, and may even cause the device to display an error
message.  But there were some older devices that would actually be
harmed by the wrong frequency signals generated by an invalid
modeline.  That, I believe, was mostly with CRT type devices, so any
digital display device on HDMI or DVI should be fine.  Also, it
usually took a little time for irreparable damage to be done, so if
you do get a blank screen or some sign of trouble it is best to have a
known good modeline set up so that you can change back to it blindly,
say by hitting the up-arrow key and enter to run the previous xrandr
command line to change back to the good mode.  And in the ultimate
emergency, it is best to know where the off switch for the device is -
the one that actually turns all power off, not the one that puts it
into standby.

If you get the display completely screwed up, then it is good to be
able to ssh into the box from outside so that you can run the command
to restart X from scratch.  In Mythbuntu 14.04, that is "restart
lightdm" (restart the display manager program).  It may need an sudo
in front of the command if you do not log in as root.  Other systems
use other display managers, such as gdm, but the name usually ends in
"dm" and you can find it using a command like "ps -e | grep -i dm".